Experimental Method for the Determination of the Saturation Vapor Pressure above Supercooled Nanoconfined Liquids

Abstract

For sorption studies, the saturation vapor pressure p ₀ above an adsorbate is of great significance. For example, it is needed for the determination of the pore size distribution, the Laplace pressure, and the chemical potential. Above the bulk triple point, T ₃ ^bulk, this pressure is identical with the saturation vapor pressure above the bulk liquid. However, below T ₃ ^bulk, the correct value of p ₀(T) is controversial. Nanoconfined fluids exhibit a shift of the freezing and melting temperatures in comparison to the bulk state. Thus, the adsorbed fluid is supercooled in a certain temperature range below T ₃ ^bulk. Here, we show that it is possible to determine the appropriate saturation vapor pressure above the nanoconfined supercooled liquid experimentally. For this purpose, we have performed sorption measurements with liquid argon in nanoporous Vycor glass in the temperature range of the supercooled liquid and at temperatures above the bulk triple point. In order to determine the unknown and temperature-dependent saturation vapor pressure of the supercooled confined adsorbate, p ₀(T), we use the Kelvin equation relating this quantity to the pore radius, r _P(p ₀), that is independent of temperature. The knowledge of the absolute values for the liquid-vapor surface tension of the supercooled adsorbate, γ_lv(T), is not required. However, we presuppose that its dependence on the unknown vapor pressure, γ_lv(p ₀), is bulk-like. Our results indicate that the saturation vapor pressure above the supercooled nanoconfined liquid corresponds to that above supercooled bulk argon (i.e., to the pressure obtained by an extension of the usual vaporization curve to T < T ₃ ^bulk). We expect that this method can be used for the determination of p ₀ above other supercooled adsorbates.